Short answer:
Virgin Mobile
Long answer:
A nice old thread, my first mobile phone provider in the UK was Cellnet in the mid 1990s in the days when people still showed off, posing with mobiles. I paid £17.99 a month back then on what was the cheapest available contract before pay as you go even existed plus £2.99 for insurance that was compulsory for the first month and I still had to pay a £35 connection charge on top, then calls still cost a whopping 50p a minute peak and 20p a minute off peak (which was after 7pm, not 6pm) to landlines or other Cellnet mobiles with no inclusive calls what-so-ever, then if you wanted to call other mobile networks you'd need to think about a getting a mortgage to pay your bill. A little while later standard call charges were at least reduced to 40p a minute peak and 12p a minute off peak. You could forget about SMS text messages, they were barely used or heard of back then, they were available on digital phones that were quite new then, but only to the same network. I even paid over £200 for the mobile phone itself which would have cost around £400 without the contract that tied me in for a minimum of 18 months and there was a strict credit check, there was no inclusive phones on contract back then and that was a "cheap" entry level phone. Back in those days it was rare to be-able to use a mobile phone indoors because the signal was just too weak even with a newer digital mobile and you'd always be looking at the signal in different areas because drop outs were very common, especially if you travelled out of the city. Times have certainly changed.
A few years later Cellnet merged and changed to BT Cellnet and now they're O2, but I had such an awful service being overcharged on my bills with nothing but issues that I changed to Orange who were much better, but in the last 10 years I've been with Virgin Mobile since I have Virgin Media cable including phone, TV and broadband.
PS: I remember when the very first ever pay as you go mobile phone came out in the UK in 1997, here was the "awful" deal, but back then it actually wasn't that bad considering the cost of contracts and mobiles phones back then, although the phone itself was pretty dire:
"One2One - Up2You
Up2You gave you a mobile phone in a box with no contract. It cost a whopping £179.99. Top-ups cost £20 and you needed to buy one every ninety days. Call charges were high at 50p per minute for any time of day.
There was just one choice of handset, the
Nortel m900, which became the UK's first GSM PAYG mobile phone.
"
Above quoted from
History of PAYG mobile phones
Even worse One2One (now T-Mobile) had by far the worst reception back then and often you were lucky just to be-able to make a call, plus you suffered frequent drop outs on the move.